Adding extra errors while transmission

Hi!

I want to get forced error after demodulating a data packet.

I am using .txt>frame>mod>channel_model>usrp_sink

usrp_source>demod>deframe>file sink

But despite of multiple complex taps in channel model am not getting
much
error in received file(basically transmitting 010101 sequence).

If I go for increasing the noise after 0.01 the demodulation does not
gives
any output!

Any way or may be I am not using the channel model so wisely!

Jay Prakash